The default database is the one assigned to a new crossword. When you create your own clues (rather than using someone else's as with the American database), you can save them to default DB or create your own DB. The reason it's empty is because you haven't saved any clues to it.

I've been using CC for over 15 years now and never had any performance issues, so I suspect any problems you are having relate to the inputs, that is, your grid structure, the wordlist you have selected and your grid-filling options. CC comes with various components (grids, wordlists, DBs) but they are really a starting point and their make-up and function should be understood before any serious work is done. Happy to help if you let me know your CC setup for grid-filling.

I'm pretty sure the performance issues I was having related to the grids I was fooling around with. It seems like CC either does the fill quickly or bangs its head against the wall for what might be a very long time.

At this point, I've solved thousands of puzzles but am just getting started trying to construct. I've been using autofill a lot, just looking at the results. Informative, in a way. If you just let it rip on a blank grid, in my observation you can expect a few clinkers (urine), maybe some unacceptable dupes (reopen and opens, e.g.), and a very boring puzzle, not worth any effort at cluing. Maybe if you were willing to look at hundreds of autofilled grids, you might find one decent one, but that doesn't seem like any fun at all, and it would of course be themeless. From what I gather, themeless puzzles are a rather small percentage of what gets published.

I think it's a very hit-and-miss approach doing it that way. If you have the standard version of CC, then you've still got a couple of things you can do: seed the crossword with theme words and get CC to fill in the rest; check the "minimum word score" used for grid filling - if it's set right with the correct wordlist, you shouldn't get any "clinkers"; right click on any slot to see what words will fit and pick a suitable one (ensure you have "find alternative word that would fit" set for AutoFind). You can build a crossword up slowly but surely this way, but you will hit dead-ends even with the intersection button set. The best option is to buy the Pro Grid Filler - not a cheap option but worth every cent for someone who is semi-serious. It gives you theme list inclusion, dictionary lookup as you go, overlapping word exclusion (solving the reopen and opens issue), priority word choice etc etc.